Circuit board including stubless signal paths and method of making same

ABSTRACT

A circuit board may include first and second sides, a plurality of circuit board layers between the sides, and a plurality of signal traces located in respective circuit board layers. The circuit board layers and the signal traces may extend from a first component connection region at the first side of the circuit board to a second component connection region at the first side of the circuit board. The signal traces may thus form stubless signal paths through the circuit board between the component connection regions. Of course, many alternatives, variations, and modifications are possible without departing from this embodiment.

FIELD

The present disclosure relates to circuit boards including signal paths,and more particularly, relates to circuit boards including stublesssignal paths.

BACKGROUND

In a computer system, printed circuit boards may include conductivepaths that electrically connect electronic components, such as chippackages and connectors, mounted to the circuit boards. The conductivepaths may include signal traces that extend along the circuit board tocarry data signals between the electronic components. In current circuitboard designs with many connections between electronic components (e.g.,chip-to-chip connections), multi-layer printed circuit boards includemultiple signal traces. Signal paths sometimes must travel from acomponent on the top of the circuit board to the signal traces in thevarious layers inside of the circuit board.

Access to the inner layers in a multi-layer printed circuit board may beprovided by vias, such as plated through-hole vias (PTHs). Vias may beformed by drilling through the circuit board and coating the innersurface with a conductive material. According to existing processes, ahole may be drilled through the entire board, even if a targeted layer(i.e., a signal trace) is located inside the board, leaving an unusedportion of the via (referred to as a via stub).

Via stubs may be problematic because they can be a source of resonancein the signal path, which causes extra signal loss. Faster signals withhigher frequency content (e.g., greater than about 4 Gb/s or 2 GHz) mayhave increasing loss due to the resonance in via stubs. Circuit boardshaving a higher stackup (e.g., in multi-CPU servers) may experience moreresonance even at lower frequencies because longer stubs reflect moreenergy. Multiple stubs within a channel (i.e., between two electroniccomponents) may produce re-reflections causing noise or interferencewith the signal transmitted by the channel.

BRIEF DESCRIPTION OF DRAWINGS

Features and advantages of the claimed subject matter will be apparentfrom the following detailed description of embodiments consistenttherewith, which description should be considered with reference to theaccompanying drawings, wherein:

FIG. 1 is a side schematic view of a circuit board including stublesssignal paths, consistent with one embodiment of the present disclosure;

FIGS. 2A-2C are a side schematic views illustrating a method of making acircuit board including stubless signal paths, consistent with oneembodiment of the present disclosure;

FIG. 3 is a side schematic view of an electronic component coupled tosignal traces in a circuit board, consistent with one embodiment of thepresent disclosure, illustrating a shift in horizontal position of theendpoints of the signal traces;

FIG. 4 is a schematic plan view of a circuit board including stublesssignal paths providing a chip-to-chip link, consistent with oneembodiment of the present disclosure;

FIG. 5 is a schematic plan view of a circuit board including cutoutregions around the ends of the stubless signal paths, consistent withone embodiment of the present disclosure;

FIG. 6 is a schematic cross-sectional side view of the cutout regionsshown in FIG. 5; and

FIG. 7 is a side schematic view of a computer system including a motherboard including stubless signal paths, consistent with anotherembodiment of the present disclosure.

Although the following Detailed Description will proceed with referencebeing made to illustrative embodiments, many alternatives,modifications, and variations thereof will be apparent to those skilledin the art. Accordingly, it is intended that the claimed subject matterbe viewed broadly.

DETAILED DESCRIPTION

Referring to FIG. 1, a circuit board 100 may include first and secondsides 102, 104 and a plurality of circuit board layers 110 between thefirst and second sides 102, 104. The circuit board layers 110 mayinclude signal traces 112 a-112 c electrically connecting electroniccomponents 106, 108 mounted to at least one side 102 of the circuitboard 100. The circuit board layers 110 and the signal traces 112 a-112c extend from one component connection region at the first side 102(e.g., from component 106), through the circuit board 100, to anothercomponent connection region at the first side 102 (e.g., to thecomponent 108). Because the signal traces 112 a-112 c extend directly tothe side 102 of the circuit board 100, vias are not necessary to connectthe electronic components 106, 108 to the signal traces 112 a-112 c. Thesignal traces 112 a-112 c thus provide stubless signal paths between theelectronic components 106, 108.

In one embodiment, the circuit board layers 110 may be made of adielectric material and the signal traces 112 a-112 c may be made ofcopper, as described in greater detail below. The circuit board layers110 may also include ground paths 114 a-114 c (e.g., ground planes)spaced from the respective signal traces 112 a-112 c. The ground paths114 a-114 c may also extend generally from one component connectionregion to another component connection region of the circuit board 100.The ground paths 114 a-114 c may serve as ground reference planes andmay be coupled together using vias, such as plated through-hole vias orstubless vias. The circuit board 100 may further include electricalcontacts 116 a-116 c and 118 a-118 c, such as contact pads, at the endpoints of the signal traces 112 a-112 c. The electrical contacts 116a-116 c and 118 a-118 c are located on the side 102 of the circuit board100 to provide electrical contact between the electronic components 106,108 and the respective signal traces 112 a-112 c.

The stubless signal paths provided by the signal traces 112 a-112 c mayform communication channels between the electronic components 106, 108.One embodiment of the electronic component 106 may include a processorchip 130 and a chip package/substrate 132. One embodiment of theelectronic component 108 may include an electrical connector 140configured to connect the circuit board 100 to another component orcircuit board. In other embodiments, the signal traces 112 a-112 c maybe used to connect two chip packages (e.g., a chip-to-chip connection),two electrical connectors, and/or any other combination of electroniccomponents capable of being coupled to a circuit board. The electroniccomponents 106, 108 may include solderable pads for electricallyconnecting to the electrical contacts 116 a-116 c and 118 a-118 c. Inaddition to solderable pads, the electronic components 106, 108 may beelectrically connected using other techniques, such as non-soldered orpartially soldered spring contacts.

For purposes of clarity, FIG. 1 is not drawn to scale and shows onlythree layers 110 including three signal traces 112 a-112 c. Thoseskilled in the art will recognize that a circuit board includingstubless signal paths may be constructed, according to the methodsdescribed below, to have different numbers of layers and signal traces.The circuit board may also have different configurations and sizes knownto those skilled in the art, and the signal traces may have differentlengths and routes through the circuit board depending upon thelocations of the electronic components being connected. A circuit boardconsistent with the present disclosure may also include signal pathsincluding conventional vias (e.g., with stubs) in addition to thestubless signal paths. In one embodiment, for example, stubless signalpaths may be used for high speed signals (e.g., greater than about 4Gb/s) or other signal paths that may be susceptible to resonance andconventional signal traces with vias may be used for non-critical signalpaths (e.g., power grid connections or low speed control signals).

Referring to FIGS. 2A-2C, one method of making a circuit board includingstubless signal paths is described in greater detail. The method mayinclude providing a plurality of flexible circuit layers 210 a-210 cincluding a dielectric material 211 a-211 c and signal traces 212 a-212c formed on or in the dielectric material 211 a-211 c. The flexiblecircuit layers 210 a-210 c may also include ground paths 214 a-214 c. Aflexible circuit layer has sufficient flexibility to allow portions ofthe circuit layer to be deformed and raised to one side of the circuitboard, as described in greater detail below. In one example, theflexible circuit layer includes at least portions with sufficientflexibility to allow a deformation angle of about 45° or less. Theflexible circuit layers 210 a-210 c may be flexible only in the portionsthat need to be deformed (e.g., at the ends) or may be flexible alongthe entire length of the flexible circuit layers 210 a-210 c. Althoughthe top layer is described as a flexible circuit layer 210 a, this layer210 a may not need to deform and thus may not be flexible in someembodiments.

In one embodiment, the flexible circuit layers 210 a-210 c may besimilar to segments of flexible interconnect (also referred to asflexible printed wire or flex) known to those skilled in the art. Thedielectric materials used in the flexible circuit layers 210 a-210 c mayinclude any dielectric materials capable of providing the desiredflexibility or deformation angle. The dielectric material providedbetween the signal traces 212 a-212 c and the ground paths 214 a-214 cmay also be relatively incompressible to maintain acceptable signaltrace-ground plane spacing. Those skilled in the art will recognize anacceptable range of compressibility. To provide the desired flexibilityand incompressibility, the flexible circuit layers 210 a-210 c may havea standard flame retardant type 4 (FR-4) construction including adielectric between the signal traces 212 a-212 c and ground paths 214a-214 c of either prepreg or polyamide, or similar material such asepoxy-based materials or PTFE or similar materials. One example of apolyamide dielectric that may be used includes the polyamide flexiblelaminate available under the name Pyralux® AP from DuPont™. In oneembodiment, a single one of the flexible circuit layers 210 a-210 c mayhave a thickness in a range of about 5-20 mils, although differentstackups are possible depending on trace dimensions. The copper tracesmay be formed on or in the flexible circuit layers 210 a-210 c usingtechniques known to those skilled in the art.

The signal traces 212 a-212 c may extend from one flexible portion toanother flexible portion of the flexible circuit layers 210 a-210 c(e.g., from one end to another end). The flexible circuit layers 210a-210 c may also include electrical contacts 216 a-216 c and 218 a-218c, such as contact pads, at the end points of the signal traces 212a-212 c to provide electrical contact to the respective signal traces212 a-212 c. The electrical contacts 216 a-216 c and 218 a-218 c may beformed using conductive material connected to the traces 212 a-212 c,for example, after the lamination process. Alternatively, the electricalcontacts 216 a-216 c may be ends of the traces 212 a-212 c, which may beexposed, for example, by creating an opening in the top cover layer ofthe dielectric. The ends of the flexible circuit layers 210 a-210 c maybe staggered to allow the electrical contacts 216 a-216 c at the ends ofthe signal traces 212 a-212 c to be moved or raised to one side of thecircuit board, as described below. In the exemplary embodiment, theflexible circuit layers 210 a-210 c have different sizes and arepositioned in a layered arrangement according to size (e.g., withsmaller sized circuit board layers on top of larger sized circuit boardlayers), which results in staggering of the ends of the flexible circuitlayers 210 a-210 c.

When the flexible circuit layers 210 a-210 c are stacked in the layeredarrangement, as shown in FIG. 2B, the flexible portions (e.g., the ends)of the flexible circuit layers 210 a-210 c may be moved or raised towardone side 202 of the stack or layered arrangement. In one embodiment,shims 220, 222 may be used to move the flexible portions of the flexiblecircuit layers 210 a-210 c toward the side 202 of the stack. In oneembodiment, the shims 220, 222 may have an angled surface with an anglecorresponding to a desired deformation angle of the flexible circuitlayers 210 a-210 c, for example, less than about 45°. Other shapes andconfigurations of the shims 220, 222 may be used depending upon thedesired deformation and alignment of the electrical contacts 216 a-216 cand 218 a-218 c. The shims 220, 222 may be made of the same dielectricmaterial (e.g., polyamide) as used for the circuit board layers 210a-210 c.

When the flexible circuit layers 210 a-210 c are stacked with theflexible portions raised toward the one side 202 and the electricalcontacts 216 a-216 c and 218 a-218 c properly aligned, the flexiblecircuit layers 210 a-210 c may be laminated together. To laminate theflexible circuit layers 210 a-210 c, pressure and heat may be applied tothe stack until the flexible circuit layers 210 a-210 b bond withadjacent layers. The laminated flexible circuit layers 210 a-210 c formthe circuit board 200, as shown in FIG. 2C. In one embodiment, pressuremay be applied in a range of about 200 psi and heat may be applied in arange of about 150-200° C. using lamination equipment and techniquesknown to those skilled in the art. Other pressures and temperatures maybe used depending upon the materials used. The shims 220, 222 may alsobe laminated together with the flexible circuit layers 210 a-210 c.Heating and pressing may also be performed such that some melting occursin the dielectric material to facilitate planarizing the circuit board.

The resulting circuit board 200 may have a thickness t in a range ofabout 50-60 mils and a spacing s between the electrical contacts 216a-216 c and 218 a-218 c in a range of about 40-60 mils. The spacing andpositioning of the electrical contacts 216 a-216 c and 218 a-218 cgenerally correspond to the spacing and positioning of the matingcontacts (e.g., solderable pads) on a corresponding electronic componentto be mounted to the circuit board 200. To provide a spacing andpositioning that corresponds to an electronic component, the electricalcontacts 216 a-216 c and 218 a-218 c may be aligned when the flexiblecircuit layers 210 a-210 c are stacked in the layered arrangement. Thisalignment of the electrical contacts 216 a-216 c and 218 a-218 c mayaccount for any shifting caused by deformation of the flexible circuitlayers 210 a-210 c, as described below.

When the flexible circuit layers are pressed to raise the flexibleportions to one side, the deformation of the flexible circuit layerscauses the horizontal position of some of the electrical contacts 316a-316 c to shift, as shown in FIG. 3. The electrical contact 316 d ofthe signal trace 312 d shifts horizontally by a distance of d1, theelectrical contact 316 c of the signal trace 312 c shifts horizontallyby a distance of d2, and the electrical contact 316 b of the signaltrace 312 b shifts horizontally by a distance of d3. As illustrated, theextent of the shift is greater for layers that are deformed by a greaterextent. The shift d1 of the electrical contact 316 d in the lower mostlayer, for example, is greater than the shifts d2 and d3 of theelectrical contacts 316 c,316 b in the other layers. Thus, the shiftingmay be greater for thicker stacks of flexible circuit layers. Becausethe top layer is not deflected in this embodiment, the electricalcontact 316 a of the signal trace 312 a does not shift horizontally as aresult of deflection.

These horizontal shifts may be taken into consideration when theflexible circuit layers including the signal traces 312 a-312 d arestacked. In other words, the electrical contacts 316 a-316 c are alignedwith an original spacing and positioning in the layered arrangement offlexible circuit layers such that the electrical contacts 316 a-316 c inthe resulting circuit board have a spacing and positioning correspondingto the spacing and positioning of the corresponding contacts on theelectronic component 302. The original spacing and positioning isequivalent to the desired spacing and positioning plus the horizontalshift amounts d1, d2, and d3.

In one embodiment, shown in FIG. 4, a circuit board 400 may include oneor more groups of signal traces 412 that provide stubless signal pathsbetween chips/packages 402 and/or other electronic components mounted onthe circuit board 400. In this embodiment, at least a portion of thecircuit board 400 may be formed by laminating flexible circuit layers,as described above. The flexible portions that are raised to one side ofthe circuit board 400, as described above, may be located in thecomponent connection regions of the circuit board 400 where thechips/packages 402 or other electronic components are to be mounted. Inthe illustrated embodiment of a multiple chip circuit board 400, thesignal traces 412 provide stubless signal paths in multiple transversedirections, for example, with one group of signal traces 412 extendingin the X direction and another group of signal traces 412 extending inthe Y directions. Signal traces may also provide stubless signal pathsalong a single dimension (e.g., for a single chip-to-chip link).Although the signal traces 412 are shown as straight in the illustratedembodiment, the signal traces 412 may also be routed in differentdirections.

Referring to FIGS. 5 and 6, one method of constructing a circuit board500 capable of providing multiple chip or component connections isdescribed in greater detail. The circuit board 500 may be constructed byproviding flexible portions 506 to be deformed or raised in thecomponent connection regions. Signal traces 512 in the circuit board mayextend between the flexible portions 506. In this embodiment, thecircuit board 500 may include cutout regions 508 around the flexibleportions 506 such that deformation of the flexible portions 506 islocalized. Localizing the deformation facilitates formation of signaltraces 512 in transverse directions (e.g., in both the X and Ydirections) with proper alignment of the electrical contacts.

As shown in FIG. 6, one or more shims 520 may be used to raise theflexible portions 506 to one side of the circuit board 500, as describedabove. The flexible portions 506 being raised include the electricalcontacts 516 a-516 c connected to the signal traces 512 a-512 c in therespective flexible circuit layers 510 a-510 c. The cutout region 508allows the flexible portions of the respective flexible circuit layers510 a-510 c to be raised separately from the surrounding portion of thecircuit board 500, thereby minimizing or eliminating localized “hills”on the circuit board. In one embodiment, the entire circuit board 500may be formed of laminated flexible circuit layers. In otherembodiments, one or more layers or portions of the circuit board 500 maybe formed of other circuit board constructions known to those skilled inthe art. After raising the flexible portions 506 with the electricalcontacts 516 a-516 c having the desired alignment, the flexible circuitlayers 510 a-510 c may be laminated together as described above.

Referring to FIG. 7, a computer system 650, such as a personal computer,may include a mother board 600 including signal traces 612 forming oneor more stubless signal paths connecting one or more electroniccomponents, such as chips 602, 606 and/or connectors 604. The circuitboard 600 including the signal traces 612 may be constructed by forminga layered arrangement and laminating flexible circuit layers, asdescribed above. The computer system 650 may also include a chassis 652enclosing the mother board 600 and one or more computer devices 654,656, such as a hard drive and/or an optical drive.

According to alternative embodiments, a circuit board including signaltraces forming stubless signal paths, consistent with embodiments of thepresent disclosure, may be used in modular platforms, such as anadvanced telecommunications computing architecture (Advanced TCA orATCA) system. Circuit boards including signal traces forming stublesssignal paths, consistent with embodiments of the present disclosure, mayalso be used in servers, mobile products and consumer products.

Consistent with one embodiment, an apparatus includes a circuit boardincluding first and second sides and a plurality of circuit board layersbetween the first and second sides. The circuit board layers include aplurality of signal traces located in respective ones of the circuitboard layers. The circuit board layers and the respective signal tracesextend from a first component connection region at the first side of thecircuit board to a second component connection region at the first sideof the circuit board. The signal traces form stubless signal pathsthrough the circuit board between the component connection regions.

Consistent with another embodiment, a method includes providing aplurality of flexible circuit layers. The flexible circuit layersinclude a dielectric material and respective signal traces extendingfrom first flexible portions to second flexible portions of therespective flexible circuit layers. The method includes positioning theflexible circuit layers in a layered arrangement such that the first andsecond flexible portions of the flexible circuit layers are raised toone side of the layered arrangement. The method further includeslaminating the flexible circuit layers together to form a circuit boardstructure having first and second sides. The laminated flexible circuitlayers form circuit board layers and the signal traces form stublesscircuit board signal paths extending from a first component connectionregion at the first side of the circuit board to a second componentconnection region at the first side of the circuit board.

Consistent with a further embodiment, a computer includes a chassis anda mother board located in the chassis. The mother board includes firstand second sides and a plurality of circuit board layers between thefirst and second sides. The circuit board layers include a plurality ofsignal traces located in respective ones of the circuit board layers.The circuit board layers and the respective signal traces extend from afirst component connection region at the first side of the circuit boardto a second component connection region at the first side of the circuitboard. The signal traces form stubless signal paths through the circuitboard between the component connection regions. The computer furtherincludes at least first and second electronic components mounted on themother board. The first and second electronic components areelectrically connected to at least some of the signal traces at thefirst and second component connection regions, respectively.

Various features, aspects, and embodiments have been described herein.The features, aspects, and embodiments are susceptible to combinationwith one another as well as to variation and modification, as will beunderstood by those having skill in the art. The present disclosureshould, therefore, be considered to encompass such combinations,variations, and modifications.

The terms and expressions which have been employed herein are used asterms of description and not of limitation, and there is no intention,in the use of such terms and expressions, of excluding any equivalentsof the features shown and described (or portions thereof), and it isrecognized that various modifications are possible within the scope ofthe claims. Other modifications, variations, and alternatives are alsopossible. Accordingly, the claims are intended to cover all suchequivalents.

1. An apparatus comprising: a circuit board including first and secondsides and a plurality of circuit board layers between the first andsecond sides, the circuit board layers including a plurality of signaltraces located in respective ones of the circuit board layers andextending with the circuit board layers to the first side of the circuitboard, the circuit board layers and the respective signal tracesextending from a first component connection region at the first side ofthe circuit board to a second component connection region at the firstside of the circuit board, by extending to the first side of the circuitboard the signal traces form stubless signal paths through the circuitboard between the component connection regions without using vias toconnect the component connection regions to the signal traces.
 2. Theapparatus of claim 1 further comprising contact pads in the componentconnection regions at the first side of the circuit board, the signaltraces in the circuit board layers extending from and electricallyconnected to respective ones of the contact pads.
 3. The apparatus ofclaim 1 further comprising electronic components mounted to the circuitboard and electrically connected to at least some of the signal tracesat the component connection regions.
 4. The apparatus of claim 1 furthercomprising at least two groups of the signal traces.
 5. The apparatus ofclaim 1 wherein the circuit board layers include a plurality of groundpaths located in respective ones of the circuit board layers.
 6. Theapparatus of claim 1 wherein the circuit board layers include aplurality of laminated flexible circuit layers.
 7. The apparatus ofclaim 1 wherein the circuit board layers include layers of dielectricmaterial.
 8. The apparatus of claim 1 wherein said circuit board is amother board.
 9. The apparatus of claim 3 wherein the electroniccomponents include multiple chips.
 10. The apparatus of claim 4 whereinthe two groups of the signal traces extend in directions that aretransverse to one another.
 11. The apparatus of claim 7 wherein thesignal traces include copper traces formed on or in the dielectricmaterial.
 12. The apparatus of claim 7 wherein the dielectric materialincludes at least one of a dielectric material selected from the groupconsisting of polyimide and prepreg.